An optical disk has been well known as a recording medium for use in an optical disk apparatus. The surface of the optical disk is spirally or concentrically provided with a plurality of tracks. When the optical disk apparatus is loaded with the optical disk, a light beam is led so as to be converged onto the track by an optical system of an optical pickup, thereby forming a light spot on the track. According to the projection of the light spot, the recording, erasing, and reproducing of information is carried out with respect to a target track.
The light spot should be converged on the desired track in accuracy in order that the above-mentioned optical apparatus carries out recording, erasing, and reproducing of information respectively in accuracy. To meet this requirement, the optical disk apparatus is generally arranged such that (1) a focussing control by which the light spot can accurately be focussed on the track and (2) a tracking control by which the light spot can accurately track the desired track are concurrently carried out.
A galvanic mirror may be used as a means for changing the position of the light spot by the use of the above mentioned tracking. The following deals with a magneto-optical disk apparatus having a separate-type pickup, which is a conventional optical disk apparatus, with reference to FIGS. 6 and 7.
FIG. 7 shows a schematic diagram showing the magneto-optical disk apparatus having the separate-type pickup. As shown in FIG. 7, a magneto-optical disk 51 is controlled by a motor 52 so as to rotate at either a predetermined linear velocity of a predetermined angular velocity. An optical head 53 leads the light beam onto the magneto-optical disk 51 through an object glass 58 during recording, erasing, and reproducing of information. The optical head 53 also converges the reflected light of the magneto-optical disk 51 through the object glass 58. The reflected light bears a reproduced signal therein. During recording or erasing, a coil 54 applies a magnetic field to a surface of the magneto-optical disk 51 which is opposite to the other surface where the light beam is projected. The magnetization direction of a magnetic layer is reversed by the magnetic field application of the coil 54. The magnetic layer is located in the substantially middle of the magneto-optical disk 51 (see FIG. 7). Note that the magnetization direction varies depending on the direction of the current applied to the coil 54.
The optical head 53 is composed of the first block and the second block. The first block is mainly provided with a galvanic mirror 56 for changing an optical path so that a light beam which is projected vertically upward (direction a of FIG. 7) from a laser light source 55 having a semiconductor laser therein directs to a direction (direction b of FIG. 7) perpendicular to direction a. The second block is mainly provided with (1) the object glass 58, (2) a mirror 57 for directing the light beam, which is changed in its optical path by the galvanic mirror 56, to the object glass 58, and (3) an object glass driving device 59.
The galvanic mirror 56 is composed of a mirror section and a mirror driving section. The mirror driving section is arranged so as to rotate the mirror section around an axis extending to the direction perpendicular with respect to the paper surface of FIG. 7. With the arrangement, a fine adjustment (i.e., tracking control) of the light spot formed on the magneto-optical disk 51 with respect to the radial direction of the magneto-optical disk 51, is carried out.
The second block is movable by a linear motor 50 with respect to the radial direction of the magneto-optical disk 51. The semiconductor laser of the laser light source 55 projects the laser light beam having a bad temperature characteristic. So, the automatic power control of the outgoing beam from the semiconductor laser is carried out such that the outgoing beam from the semiconductor laser becomes stable.
The following deals with the automatic power control of the outgoing beam with reference to FIG. 6.
As shown in FIG. 6, a semiconductor laser 41 is driven by collector current I.sub.a of a transistor 45 such that the light intensity of outgoing beam L.sub.b becomes strong when collector current I.sub.a increases while the light intensity of outgoing beam L.sub.b becomes weak when collector current I.sub.a decreases. The transistor 45 is driven by a differential amplifier 44. More specifically, collector current I.sub.a of the transistor 45 varies depending on an output signal of the differential amplifier 44.
Outgoing beam L.sub.b of the semiconductor laser 41 directs to a photodetector 42. The photodetector 42 outputs current I.sub.c which varies depending on the incident beam L.sub.b. Current I.sub.c is outputted to current/voltage (I/V) converting circuit 43. The current/voltage converting circuit 43 converts current I.sub.c inputted thereto into corresponding voltage V.sub.d to output thereof.
Voltage V.sub.d is outputted to a non-inverting input terminal of the differential amplifier 44. An inverting input terminal of the differential amplifier 44 is connected to a common terminal 46c of a switch circuit 46. Reference voltage e.sub.R corresponding to the reproduction operation is applied to a contact point 46b of the switch circuit 46. In contrast, reference voltage e.sub.W corresponding to the recording and erasing operations is applied to a contact point 46a of the switch circuit 46.
Note that the respective reference voltages e.sub.R and e.sub.W are set so as to satisfy the relation therebetween, e.sub.W &gt;e.sub.R. The switching of the switch circuit 46 is controlled in response to control signal f from a control device such as CPU. Moreover, an output signal of the differential amplifier 44 is connected to a base of the transistor 45 through a resistor 48 for limiting the current therethrough. An emitter of the transistor 45 is connected to power source V.sub.c through a resistor 47.
With the arrangement, the differential amplifier 44 carries out the differential amplifying with respect to voltage V.sub.d and reference voltage e.sub.W during recording and erasing of information. And, when voltage V.sub.d is greater than reference voltage e.sub.W, the output signal of the differential amplifier 44 increases, thereby increasing the base voltage of the transistor 45 and decreasing collector current I.sub.a. Accordingly, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 becomes weak and current I.sub.c decreases, thereby decreasing voltage V.sub.d. In contrast, when V.sub.d &lt;e.sub.W is satisfied, the output signal of the differential amplifier 44 decreases. And, the base voltage of the transistor 45 decreases, so collector current I.sub.a increases. Accordingly, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 becomes strong and current I.sub.c increases, thereby increasing voltage V.sub.d. By repeating the above mentioned procedures, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 is controlled such that voltage V.sub.d substantially coincides with reference voltage e.sub.W .
The differential amplifier 44 also carries out the differential amplifying with respect to voltage V.sub.d and reference voltage e.sub.R during reproducing of information. And, when voltage V.sub.d is greater than reference voltage e.sub.W, the output signal of the differential amplifier 44 increases, thereby increasing the base voltage of the transistor 45 and decreasing collector current I.sub.a. Accordingly, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 becomes weak and current I.sub.c decreases, thereby decreasing voltage V.sub.d. In contrast, when V.sub.d &lt;e.sub.R is satisfied, the output signal of the differential amplifier 44 decreases. And, the base voltage of the transistor 45 decreases, so collector current I.sub.a increases. Accordingly, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 becomes strong and current I.sub.c increases, thereby increasing voltage V.sub.d. By repeating the above mentioned procedures, the light intensity of outgoing beam L.sub.b of the semiconductor laser 41 is controlled such that voltage V.sub.d substantially coincides with reference voltage e.sub.R.
As mentioned above, the semiconductor laser 41 projects light beams having the respective predetermined light intensities of outgoing beam L.sub.b corresponding to recording, erasing, and reproducing of information. However, when the tracking control is carried out by the use of the galvanic mirror 56, there presents the problem that the light intensity of the light spot focussed on the optical disk fluctuates, due to the fact that the light beam is kicked off by the object glass 58 when the galvanic mirror 56 rotates, though it is provided that the light intensity of the outgoing beam is kept constant. The following deals with the fluctuation of the light intensity of the light spot focussed on the optical disk with reference to FIG. 8.
When a light beam, which has the distribution (Gaussian distribution) of the outgoing light beam intensity with respect to the cross-sectional direction like FIG. 8(b), is directed to the galvanic mirror 56, the light beam is reflected like the solid line of FIG. 8(a) and thereafter is converged by the object glass 58 onto the optical disk (not shown), thereby forming a ;light spot a on the optical disk. Note that cross-sectional area Pa of the slanting line of FIG. 8(b) corresponds to the light intensity of the light spot a. In that case, the rotation angle of the galvanic mirror 56 with respect to the vertical direction is expressed as .theta.a.
Meanwhile, it is assumed that the rotation angle decreases by .DELTA..theta. and becomes .theta.b after the galvanic mirror 56 rotates in accordance with the tracking control. In that case, the light beam is reflected by the galvanic mirror 56 (see the dashed line of FIG. 8(a)) and thereafter the light beam is converged onto the optical disk so as to form a light spot b through the object glass 58. Note that cross-sectional area Pb of the dashed slanting line of FIG. 8(c) corresponds to the light intensity of the light spot b.
Accordingly, though the outgoing light beam intensity of the semiconductor laser is stabilized like above by the automatic power control, the light intensity of the light spot formed on the optical disk changes according to the change of the rotation angle of the galvanic mirror 56 due to the tracking control operation. Therefore, recording, erasing, and reproducing of information can not be achieved under the projection condition of the optimum light spot, thereby resulting in the serious problem that the reliability with respect to the recorded information decreases and thereby resulting in the serious problem that information can not perfectly erased during erasing operation.